winsys/amdgpu: don't assume GTT if the VRAM flag isn't set
winsys/amdgpu: don't assume GTT if the VRAM flag isn't set

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler